1. Field of the Invention
The present invention relates to a circuit including a choke coil, and more particularly, to a circuit having a choke coil inserted into a signal line having communication and power-provision functions, and also relates to a choke coil.
2. Description of the Related Art
In the related art, differential transmission circuits are used for communication. In differential transmission, a twisted pair line carries signals having opposite phases, and the high/low level is determined based on which signal line has higher potential. For example, the current most common LAN standard for personal computers is Ethernet (registered trademark), and a pulse transformer is provided as an interface thereof. If high noise radiation is produced from a cable, common-mode choke coils are used before and after the pulse transformer.
One advantage of using a common-mode choke coil is that a restriction effect acts on common-mode noise without affecting the signals carried with opposite phases on the twisted pair line. In differential transmission, therefore, currents having the same magnitude flow with opposite phases in the twisted pair line, and the magnetic fluxes generated by the differential signal current are cancelled out in a magnetic core. On the other hand, the magnetic fluxes generated by a noise current flowing in-phase are mutually strengthened in a magnetic core.
In differential transmission communication, signals having a frequency of 100 MHz or higher may be used, and the signal frequency and the noise frequency band often overlap each other. A low-pass filter, such as a normal-mode choke coil, controls noise and signals at the same time, and is therefore difficult to use.
One known common-mode choke coil of the related art for preventing noise from entering a telephone line is described in Patent Document 1 (Japanese Unexamined Utility Model Registration Application Publication No. 4-4712). As shown in FIG. 9, a common-mode choke coil 1 includes a magnetic core having two U-shaped core members 10 and 11, two bobbins 2 and 3, and four windings 4, 5, 6, and 7.
The bobbins 2 and 3 have cylindrical body portions 2a and 3a arranged in parallel to each other. Leg portions 10b and 11b of the core members 10 and 11 are inserted through holes 2b and 3b in the cylindrical body portions 2a and 3a, respectively. The core members 10 and 11 form one closed magnetic path in which the leading ends of the leg portions 10b and 11b abut against each other in the holes 2b and 3b. 
The windings 4 and 5 are bifilar-wound in one layer on the cylindrical body portion 2a of the bobbin 2. The windings 6 and 7 are also bifilar-wound in one layer on the cylindrical body portion 3a of the bobbin 3. The windings 4 to 7 are wound so as to mutually strengthen magnetic fluxes in the magnetic core when an in-phase current flows.
In the common-mode choke coil 1 having this structure, the number of winding portions in which the windings 4 and 5 or the windings 6 and 7 are adjacent is only two in the horizontal direction shown in FIG. 9, and the stray capacitances caused at the adjacent wound portions are connected in series a number of times corresponding to the number of turns. Thus, the stray capacitance can be reduced, and the ability to prevent noise from entering the high band can increase.
However, the common-mode choke coil 1 described in Patent Document 1 has a so-called bifilar-wound structure in which the windings 4 and 5 or the windings 6 and 7 are alternately wound in one layer on the cylindrical body portion 2a or 3a of the bobbin 2 or 3. Thus, there is a problem in that the number of turns of the windings 4 to 7 per unit length is small, resulting in small inductance obtained compared to the size of the bobbins 2 and 3. A high-precision winding machine is required to produce such a bifilar-wound structure. However, product failure still occurs due to disordered winding. Disordered winding greatly affects the high-frequency characteristics of the product.
Recently, a standard called IEEE 802.3af has been proposed by the Institute of Electrical and Electronic Engineers. This standard defines a circuit having a power-provision circuit in a traditional differential transmission circuit, and also defines power provision via a signal line, such as a LAN cable for transmitting and receiving signals. This standard is applied to devices, such as IP phones connected to LAN cables and wireless LAN access points. When a common-mode choke coil is used for noise prevention on a signal line to be defined by this standard, the magnetic fluxes generated by a power supply current are generated in the direction in which they are strengthened in a magnetic core of the common-mode choke coil. Due to the magnetic fluxes generated by the power supply current, the magnetic flux density of the magnetic core becomes close to a saturated magnetic flux density, and the common-mode choke coil inductance is reduced. The noise prevention effect is therefore reduced. One approach for preventing an increase in the magnetic flux density is to increase the cross-sectional area of the magnetic core. However, as the size of the magnetic core increases, the product size also increases. Moreover, the cost of the magnetic core occupies the majority of the product material cost. Thus, an increase in the size of the magnetic core greatly affects the product price. If the number of turns of windings is small, small magnetic fluxes are generated in the magnetic core, and the core is less saturated. However, the inductance becomes small, and the noise prevention effect is therefore reduced.